1. Field of the Invention
The present invention relates to a rotating state detection apparatus and, more particularly, to an apparatus which photoelectrically detects a rotational speed and angle of a rotating object by utilizing a beam diffracted by a diffraction grating, and which is suitable for e.g., a rotary encoder.
2. Related Background Art
Photoelectric rotary encoders are conventionally used as a means for detecting a rotational speed and variations therein of rotating mechanisms in office equipment, such as a floppy-disk drive computer and a printer, an NC machine tool, the capstan motor of a VTR, a rotating drum, and the like.
A method which adopts a photoelectric rotary encoder employs a so-called index scale system wherein a light projection means and a light-receiving means are arranged to oppose each other with a main scale and a stationary index scale sandwiched therebetween. The main scale is formed by arranging light-transmitting and light-shielding portions at equal angular intervals at the peripheral portion of a disk coupled to a rotating shaft. The index scale is formed by arranging light-transmitting and light-shielding portions at the same angular intervals as in the main scale. According to this method, a signal in synchronism with the pitch of the adjacent light-transmitting and light-shielding portions of both scales can be obtained as the main scale is rotated. The obtained signal is subjected to frequency analysis to detect variations in the rotational speed of the rotating shaft. The smaller the pitch of the light-transmitting and light-shielding portions on both scales, the higher the detection precision. However, when the scale pitch is small, the S/N ratio of the output signal from a light-receiving means is degraded by diffracted light, resulting in low precision. In order to prevent this, if a total number of light-transmitting and light-shielding portions of the main scale is fixed, and the intervals between the two portions are increased to a point at which the light-receiving means is free from the effects of the diffracted light, the diameter and thickness of the disk of the main scale are increased, resulting in an increase in the overall apparatus size. Therefore, the object to be rotated is overloaded.